1. Field of the Invention
The invention relates to a steering control system of a vehicle, such as an automobile, and more particularly to a vehicle steering control system including a variable transmission ratio device operable to change the steering transmission ratio or gear ratio.
2. Description of the Related Art
As one type of steering control system of a vehicle, such as an automobile, a steering control system including a variable transmission ratio device operable to change the steering transmission ratio is known. This type of steering control system generally includes an input shaft that is rotatably supported and engages with the steering wheel so as not to rotate relative to the steering wheel, an output shaft that is rotatably supported, a motion conversion mechanism that converts rotary motion of the output shaft into a motion that changes the steering angle of steerable road wheels, and a variable transmission ratio device that rotates the output shaft relative to the input shaft so as to change the transmission ratio as the ratio of the amount of rotary motion of the output shaft to the amount of rotary motion of the input shaft.
While variable transmission ratio devices having various constructions have been proposed, a variable transmission ratio device including an electric motor and a speed reduction mechanism is well known. The electric motor has a stator, and a rotor that is driven by rotor driving torque generated from interactions between the rotor and the stator and is rotated relative to the stator, and the speed reduction mechanism includes a rotator that rotates along with the rotor. The variable transmission ratio device is arranged to operate in the following manner: when the rotator rotates relative to the input shaft by a first rotational angle, the output shaft rotates relative to the input shaft by a second rotational angle that is different from the first rotational angle, and, when the rotator does not rotate relative to the input shaft, the output shaft does not rotate relative to the input shaft.
While control current needs to be supplied to the electric motor so as to control the variable transmission ratio device, a spiral cable for supplying control current from the vehicle body side to the electric motor is needed in the case where the electric motor is incorporated in the steering column. To eliminate the need for the spiral cable, there is also known a variable transmission ratio device constructed such that the stator is fixed to the vehicle body and control current is supplied to the stator.
In the steering control system having the variable transmission ratio device, a locking device is provided so that the variable transmission ratio device performs control for changing the transmission ratio with reliability when the variable transmission ratio device is in a normal operating condition, and so that the rotary motion and torque are surely transmitted between the input shaft and the output shaft when an abnormality occurs in the variable transmission ratio device, as disclosed in, for example, Japanese Patent Application Publication No. 2005-162124 (JP-A-2005-162124) and Japanese Patent Application Publication No. 2005-14680 (JP-A-2005-14680). The locking device is switched between a lock-off state that allows changes in the transmission ratio between the input shaft and the output shaft, and a lock-on state that inhibits changes in the transmission ratio between the input shaft and the output shaft.
As one type of speed reduction mechanism of the variable transmission ratio device, a strain wave gearing mechanism (as disclosed in, for example, JP-A-2005-14680) is well known. This mechanism has a first ring gear member that rotates with the input shaft, a second ring gear member that rotates with the output shaft, and a band-like external-teeth gear member that is mounted around an elliptic rotator that rotates with the rotor of the electric motor, and is rotatable relative to the rotator. The external-teeth gear member meshes with the first and second ring gear members at positions corresponding to the long diameter of the ellipse of the rotator. The number of teeth of the external-teeth gear member is equal to that of one of the first and second ring gear members, and is different from that of the other of the first and second ring gear members. In operation, the rotator is driven by the electric motor and rotated relative to the input shaft, so that the transmission ratio is changed.
Generally, the locking device of the steering control system having the strain wave gearing mechanism as described above, when placed in the lock-on state, inhibits relative rotation of the rotor and the stator so as to inhibit rotation of the rotator, thereby to inhibit changes in the transmission ratio between the input shaft and the output shaft. When the locking device is in the lock-on state, the rotator does not rotate, and rotation of the first ring gear member due to rotation of the input shaft causes the external-teeth gear member to be rotated around the rotator. The rotation of the external-teeth gear member is transmitted to the output shaft via the second ring gear member, so that the rotary motion and torque of the input shaft are transmitted at a given gear ratio to the output shaft. Conversely, rotation of the second ring gear member due to rotation of the output shaft causes the external-teeth gear member to be rotated around the rotator, and the rotation of the external-teeth gear member is transmitted to the input shaft via the first ring gear member, so that the rotary motion and torque of the output shaft are transmitted at a given gear ratio to the input shaft.
In the steering control system of the type in which the stator of the electric motor of the variable transmission ratio device is fixed to the vehicle body, if the external-teeth gear member becomes not able to rotate relative to the rotator due to, for example, entry of foreign matter, the external-teeth gear member cannot rotate at all when the locking device is switched to the lock-on state in which rotation of the rotor and the rotator is inhibited. As a result, the rotary motion and torque cannot be transmitted between the input shaft and the output shaft, and it becomes impossible to perform turning operations on the steering wheel and turn or steer the steerable road wheels.
To avoid the situation as described above, it may be necessary to provide a special safety device that permits rotation of the rotator when high torque is applied to the rotator, even if the locking device is switched to the lock-on state in a situation where the external-teeth gear member cannot rotate relative to the rotator. The provision of the safety device may inevitably complicate the structure of the steering control system or increase the cost of the steering control system.